/cc @theacodes FYR.

Because the "documentation root" changes for inclusions between the monorepo docs build and the per-package docs build, we have to do this in two phases.

Steps before the split:

• For packages whose canonical docs are not symlinks into their own docs/ tree, move their contents to that directory and set up symlinks. Test that top-level docs still generate correctly via symlinks.
• For all packages, harmonize / de-duplicate their README.rst with the docs/index.rst landing page (see PR Docs: Replace links to '/stable/' with '/latest/'. #5901 / commit 26efddf for recently-generated packages).
• If landing page is renamed (e.g. from usage.rst -> index.rst), leave behind a redirect page (see bigquery/docs/usage.html) and link it in docs/conf.py.
• Remove any generation / mutation of README.rst / docs/index.rst from synth.py.

GA packages:

• datastore (PR Prep datastore docs for repo split. #5919)
• storage (PR Prep storage docs for repo split. #5923)
• language (PR Prep language docs for repo split. #5932)
• spanner (PR Prep spanner docs for repo split. #5938)
• translate (PR Prep translate docs for repo split. #5941)
• logging (PR Prep logging docs for repo split. #5943)

Beta packages:

• bigquery (PR Prep bigquery docs for repo split #5955)
• firestore (PR Prep firestore docs for repo split. #6000)
• pubsub (PR Prep pubsub docs for repo split. #6001)
• speech (PR Speech: Prep docs for repo split. #6017)
• videointelligence (PR Videointelligence: Harmonize / DRY 'README.rst' / 'docs/index.rst'. #6002)
• vision (PR Vision: Harmonize / DRY 'README.rst' / 'docs/index.rst'. #6003)

Alpha packages:

• asset (PR Docs: Replace links to '/stable/' with '/latest/'. #5901)
• automl (PR AutoML: Harmonize / DRY 'README.rst' / 'docs/index.rst'. #6005)
• bigquery_datatransfer (PR BigQuery Data Transfer: Harmonize / DRY 'README.rst' / 'docs/index.rst'. #6013)
• bigtable (PR Bigtable: prep docs for repo split. #6014)
• container (PR Container: harmonize / DRY 'README.rst' / 'docs/index.rst'. #6018)
• dataproc (PR Dataproc: harmonize / DRY 'README.rst' / 'docs/index.rst'. #6019)
• error_reporting (PR Error_reporting: prep docs for repo split. #6155)
• dlp (PR Docs: Replace links to '/stable/' with '/latest/'. #5901)
• dns (PR DNS: Prep docs for repo split. #6020)
• iot (PR Docs: Replace links to '/stable/' with '/latest/'. #5901)
• kms (PR KMS: harmonize / DRY 'README.rst' / 'docs/index.rst'. #6021)
• monitoring (PR Monitoring: harmonize / DRY 'README.rst' / 'docs/index.rst'. #6156)
• redis (PR Docs: Replace links to '/stable/' with '/latest/'. #5901)
• resource_manager (PR Resource-manager: prep docs for repo split. #6022)
• runtimeconfig (PR Runtimeconfig: prep docs for repo split. #6023)
• tasks (PR Docs: Replace links to '/stable/' with '/latest/'. #5901)
• texttospeech (PR Docs: Replace links to '/stable/' with '/latest/'. #5901)
• trace (PR Trace: Prep docs for repo split. #6024)

Not-yet-released packages:

• oslogin (PR Docs: Replace links to '/stable/' with '/latest/'. #5901)

After the split:

• Add docs/conf.py to all packages which don't have it (fix up bitrot for those which do).
• Add a nox -e docs target for all packages and test.
• Sync any redirect pages added in umbrella docs/conf.py with local docs/conf.py.